Crosslinking rubber composition and crosslinked rubber production method

ABSTRACT

A cross-linked rubber is excellent in both fuel oil resistance and cold resistance. A cross-linkable rubber composition contains: a nitrile group-containing copolymer rubber [A] including less than 30 wt % α,β-ethylenically unsaturated nitrile monomer units (a1), conjugated diene monomer units (b1), and α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units (d1), wherein at least part of the conjugated diene monomer units (b1) is hydrogenated; a nitrile group-containing copolymer rubber [B] including not less than 30 wt % α,β-ethylenically unsaturated nitrile monomer units (a2) and conjugated diene monomer units (b2), wherein at least part of the conjugated diene monomer units (b2) may be hydrogenated; and an amine-based cross-linking agent.

TECHNICAL FIELD

The present invention relates to a (i) cross-linkable rubber compositionand a (ii) method for producing a cross-linked rubber.

BACKGROUND ART

In general, a nitrile group-containing highly saturated copolymer rubbertypified by a hydrogenated acrylonitrile-butadiene copolymer rubber issuperior in heat resistance, ozone resistance, and other properties to atypical nitrile group-containing copolymer rubber such as anacrylonitrile-butadiene copolymer rubber. For this reason, the nitrilegroup-containing highly saturated copolymer rubber is used for a widevariety of automotive parts such as fuel oil hoses, O-rings, and in-oilbelts.

O-rings incorporated into, for example, automotive engines are used inenvironments where the O-rings are subjected to fuel oils such asgasoline. Thus, there has been a demand for O-rings having fuel oilresistance to such a degree that they do not swell in contact with thefuel oils. For example, Patent Literature 1 has proposed, as a rubbercompound that meets such a demand, a polymer blend containing (i) atleast one hydrogenated nitrile rubber, (ii) at least one hydrogenatedterpolymer rubber, (iii) at least one salt of a strong base and a weakacid comprising a group I metal, and (iv) at least one olefin/vinylacetate and/or olefin/acrylate rubber.

Furthermore, O-rings incorporated into, for example, automotive enginesused in cold climate areas are required to have cold resistance to sucha degree that they have rubber elasticity in low-temperatureenvironments. Providing O-rings having both cold resistance and fuel oilresistance has become a challenge to be addressed. It is generally knownthat, as a nitrile content of a nitrile-containing highly saturatedcopolymer rubber is reduced for the purpose of improving coldresistance, fuel oil resistance decreases accordingly. An approach forimproving those properties is, for example, controlling a molecularstructure of a hydrogenated nitrile rubber, as described in Non-patentLiterature 1.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication Tokukai No. 2005-29790(Publication date: Feb. 3, 2005)

Non-Patent Literature

[Non-Patent Literature 1]

The Journal of the Society of Rubber Science and Technology, Japan, Vol.75, pp. 32-36 (2002)

SUMMARY OF INVENTION Technical Problem

However, the molecular structure control described in Non-patentLiterature 1 is not technically easy and results in increase ofmanufacturing costs. In order to solve such a problem, there is a needfor a novel cross-linkable rubber composition that is obtained by amethod other than the molecular-structure controlling method and isexcellent in cold resistance and fuel oil resistance.

In view of this, it is an object of the present invention to provide across-linkable rubber composition that is excellent in both fuel oilresistance and cold resistance and has an excellent balance of fuel oilresistance and cold resistance.

Solution to Problem

As a result of diligent studies, the inventors of the present inventionaccomplished the present invention by finding that the above object isachievable with a cross-linked rubber that is obtained by cross-linkinga cross-linkable rubber composition containing: a nitrilegroup-containing copolymer rubber [A] including less than 30 wt %α,β-ethylenically unsaturated nitrile monomer units (a1), conjugateddiene monomer units (b1), and α,β-ethylenically unsaturated dicarboxylicacid monoester monomer units (d1), wherein at least part of theconjugated diene monomer units (b1) is hydrogenated; a nitrilegroup-containing copolymer rubber [B] including not less than 30 wt %α,β-ethylenically unsaturated nitrile monomer units (a2) and conjugateddiene monomer units (b2), wherein at least part of the conjugated dienemonomer units (b2) may be hydrogenated; and an amine-based cross-linkingagent.

That is, a cross-linkable rubber composition in accordance with thepresent invention contains: a nitrile group-containing copolymer rubber[A] including less than 30 wt α,β-ethylenically unsaturated nitrilemonomer units (a1), conjugated diene monomer units (b1), andα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1), wherein at least part of the conjugated diene monomer units (b 1)is hydrogenated; a nitrile group-containing copolymer rubber [B]including not less than 30 wt % α,β-ethylenically unsaturated nitrilemonomer units (a2) and conjugated diene monomer units (b2), wherein atleast part of the conjugated diene monomer units (b2) may behydrogenated; and an amine-based cross-linking agent.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain across-linked rubber that is excellent in both fuel oil resistance andcold resistance and has an excellent balance of fuel oil resistance andcold resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing results of measurements of fuel oil resistanceand cold resistance in Examples in accordance with the presentinvention.

FIG. 2 is a graph showing results of measurements of fuel oil resistanceand cold resistance in Examples in accordance with the presentinvention.

FIG. 3 is a graph showing results of measurements of fuel oil resistanceand cold resistance in Comparative Examples in accordance with thepresent invention.

DESCRIPTION OF EMBODIMENTS

<Cross-Linkable Rubber Composition>

A cross-linkable rubber composition in accordance with the presentinvention contains: a nitrile group-containing copolymer rubber [A]including less than 30 wt % α,β-ethylenically unsaturated nitrilemonomer units (a1), conjugated diene monomer units (b1), andα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1), wherein at least part of the conjugated diene monomer units (b 1)is hydrogenated; a nitrile group-containing copolymer rubber [B]including not less than 30 wt % α,β-ethylenically unsaturated nitrilemonomer units (a2) and conjugated diene monomer units (b2), wherein atleast part of the conjugated diene monomer units (b2) may behydrogenated; and an amine-based cross-linking agent. With the use ofthe cross-linkable rubber composition in accordance with the presentinvention, it is possible to obtain a cross-linked rubber that isexcellent in both fuel oil resistance and cold resistance and has anexcellent balance of fuel oil resistance and cold resistance.

[Nitrile Group-Containing Copolymer Rubber [A]]

The nitrile group-containing copolymer rubber [A] contained in thecross-linkable rubber composition in accordance with the presentinvention includes less than 30 wt % α,β-ethylenically unsaturatednitrile monomer units (a1), conjugated diene monomer units (b1), andα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1), wherein at least part of the conjugated diene monomer units (b 1)is hydrogenated.

(α,β-Ethylenically Unsaturated Nitrile Monomer Units (a1))

An α,β-ethylenically unsaturated nitrile monomer (ma1) forming theα,β-ethylenically unsaturated nitrile monomer units (a1) is not limitedto a specific one as long as it is an α,β-ethylenically unsaturatedcompound having nitrile groups. Examples of the α,β-ethylenicallyunsaturated nitrile monomer (ma1) include acrylonitrile;α-halogenoacrylonitrile such as a-chloroacrylonitrile andα-bromoacrylonitrile; α-alkylacrylonitrile such as methacrylonitrile andethacrylonitrile. Among these compounds, acrylonitrile andmethacrylonitrile are preferable, and acrylonitrile is particularlypreferable. As the α,β-ethylenically unsaturated nitrile monomer, two ormore of these compounds may be used in combination.

The content of the α,β-ethylenically unsaturated nitrile monomer units(a1) in the nitrile group-containing copolymer rubber [A] is less than30 wt % and preferably not more than 25 wt % of the total monomer unitsin the nitrile group-containing copolymer rubber [A]. Further, thecontent of the α,β-ethylenically unsaturated nitrile monomer units (a1)is preferably not less than 1 wt %, more preferably not less than 10 wt%, and further preferably not less than 15 wt %. If the content of theα,β-ethylenically unsaturated nitrile monomer units (a1) is too high, anobtained cross-linked product tends to have decreased cold resistance.If the content of the α,β-ethylenically unsaturated nitrile monomerunits (a1) is too low, an obtained cross-linked product tends to havepoor fuel oil resistance.

(Conjugated Diene Monomer Units (b1))

Examples of a conjugated diene monomer (mb1) forming the conjugateddiene monomer units (b1) include 1,3-butadiene, isoprene,2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Among these compounds,1,3-butadiene is preferable.

The conjugated diene monomer units (b1) in the nitrile group-containingcopolymer rubber [A] are contained in a state such that at least part ofthe conjugated diene monomer units (b1) is hydrogenated in the nitrilegroup-containing copolymer rubber [A]. The percentage of the content ofthe conjugated diene monomer units (b1) is preferably 20 wt % to 83.5 wt%, more preferably 30.5 wt % to 64.0 wt %, and further preferably 40 wt% to 58.5 wt % of the total monomer units in the nitrilegroup-containing copolymer rubber [A]. In a case where the percentage ofthe content of the conjugated diene monomer units (b1) is not less than20 wt %, an obtained cross-linked rubber will exhibit improvement inrubber elasticity. Meanwhile, in a case where the percentage of thecontent of the conjugated diene monomer units (b1) is not more than 83.5wt %, an obtained cross-linked rubber will exhibit improvements in heataging resistance, chemical resistant stability, and other properties.

Hydrogenation of the conjugated diene monomer units (b1) can beperformed by use of a conventionally known hydrogenation catalyst or thelike.

Although the degree of hydrogenation is not particularly limited,hydrogenation is performed so that the nitrile group-containingcopolymer rubber [A] has an iodine value of preferably not more than120, more preferably not more than 80, further preferably not more than25, and particularly preferably not more than 10. In a case where theiodine value is not more than 120, an obtained cross-linked rubber willexhibit improvements in heat resistance and ozone resistance.

(α,β-Ethylenically Unsaturated Dicarboxylic Acid Monoester Monomer Units(d1))

A monomer forming the α,β-ethylenically unsaturated dicarboxylic acidmonoester monomer units (d1) in the nitrile group-containing copolymerrubber [A] is not particularly limited as long as it is anα,β-ethylenically unsaturated dicarboxylic acid monoester monomer (md1)having one unesterified and non-substituted (free) carboxyl group.

The non-substituted carboxyl group is used principally forcross-linking. The nitrile group-containing copolymer rubber [A] has theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1), which is to be cross-linked with the amine-based cross-linkingagent. This leads to a high crosslink density, resulting in an obtainedcross-linked rubber being excellent in mechanical property such aselongation and being more excellent in compression set resistance andcold resistance.

An organic group, of an ester part of the α,β-ethylenically unsaturateddicarboxylic acid monoester monomer (md1), to be bonded to a carbonylgroup through an oxygen atom is preferably an alkyl group, a cycloalkylgroup, or an alkylcycloalkyl group. Such an organic group isparticularly preferably an alkyl group. The alkyl group that serves asthe organic group to be bonded to the carbonyl group through the oxygenatom is preferably an alkyl group having 1 to 12 carbon atoms, and morepreferably an alkyl group having 2 to 6 carbon atoms. Further, thecycloalkyl group that serves as the organic group to be bonded to thecarbonyl group through the oxygen atom is preferably a cycloalkyl grouphaving 5 to 12 carbon atoms, and more preferably a cycloalkyl grouphaving 6 to 10 carbon atoms. Still further, the alkylcycloalkyl groupthat serves as the organic group to be bonded to the carbonyl groupthrough the oxygen atom is preferably an alkylcycloalkyl group having 6to 12 carbon atoms, and more preferably an alkylcycloalkyl group having7 to 10 carbon atoms. As to the number of carbon atoms in the organicgroup to be bonded to the carbonyl group through the oxygen atom, in acase where the alkyl group has one or more carbon atoms, the cycloalkylgroup has 5 or more carbon atoms, and the alkylcycloalkyl group has 6 ormore carbon atoms, a cross-linkable rubber composition obtained byaddition of a cross-linking agent will exhibit improvement in processingstability. In a case where the organic group has 12 or less carbonatoms, a cross-linking rate will increase, and an obtained cross-linkedrubber will exhibit improvement in mechanical property.

Specific examples of the α,β-ethylenically unsaturated dicarboxylic acidmonoester monomer (md1) include: maleic acid monoalkyl esters such asmonomethyl maleate, monoethyl maleate, monopropyl maleate, and monon-butyl maleate; maleic acid monocycloalkyl esters such asmonocyclopentyl maleate, monocyclohexyl maleate, and monocycloheptylmaleate; maleic acid monoalkylcycloalkyl esters such asmonomethylcyclopentyl maleate and monoethylcyclohexyl maleate; fumaricacid monoalkyl esters such as monomethyl fumarate, monoethyl fumarate,monopropyl fumarate, and mono n-butyl fumarate; fumaric acidmonocycloalkyl esters such as monocyclopentyl fumarate, monocyclohexylfumarate, and monocycloheptyl fumarate; fumaric acid monoalkylcycloalkylesters such as monomethylcyclopentyl fumarate and monoethylcyclohexylfumarate; citraconic acid monoalkyl esters such as monomethylcitraconate, monoethyl citraconate, monopropyl citraconate, and monon-butyl citraconate; citraconic acid monocycloalkyl esters such asmonocyclopentyl citraconate, monocyclohexyl citraconate, andmonocycloheptyl citraconate; citraconic acid monoalkylcycloalkyl esterssuch as monomethylcyclopentyl citraconate and monoethylcyclohexylcitraconate; itaconic acid monoalkyl esters such as monomethylitaconate, monoethyl itaconate, monopropyl itaconate, and mono n-butylitaconate; itaconic acid monocycloalkyl esters such as monocyclopentylitaconate, monocyclohexyl itaconate, and monocycloheptyl itaconate; anditaconic acid monoalkylcycloalkyl esters such as monomethylcyclopentylitaconate and monoethylcyclohexyl itaconate.

Among these compounds, the α,β-ethylenically unsaturated dicarboxylicacid monoester monomer (md1) is preferably monopropyl maleate, monon-butyl maleate, monopropyl fumarate, mono n-butyl fumarate, monopropylcitraconate, mono n-butyl citraconate, or other monoester of adicarboxylic acid in which two carbon atoms forming an α,β-ethylenicallyunsaturated bond have each carboxyl groups, more preferably mono n-butylmaleate, monopropyl citraconate, or other monoester of a dicarboxylicacid having the two carboxyl groups at the cis positions (cisarrangement), and particularly preferably mono n-butyl maleate, from theviewpoint of allowing the advantageous effect of the present inventionto become more pronounced.

The percentage of the content of the α,β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d1) in the nitrilegroup-containing copolymer rubber [A] is preferably 4.5 wt % to 10 wt %,more preferably 6 wt % to 9.5 wt %, and further preferably 6.5 wt % to8.5 wt % of the total monomer units in the nitrile group-containingcopolymer rubber [A]. In a case where the content of theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1) is not less than 4.5 wt %, an obtained cross-linked rubber willexhibit improvements in mechanical property and compression setresistance. Meanwhile, in a case where the content of theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1) is not more than 10 wt %, a cross-linkable rubber compositionobtained by addition of a cross-linking agent will have good scorchstability, and an obtained cross-linked rubber will exhibit improvementin fatigue resistance.

(Other Monomer Units)

The nitrile group-containing copolymer rubber [A] may contain othermonomer units copolymerizable with the α,β-ethylenically unsaturatednitrile monomer (ma1), the conjugated diene monomer (mb1), and theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer (md1).

Examples of a monomer forming the other monomer units include(meth)acrylic acid alkoxyalkyl ester monomer, (meth)acrylic acid alkylester monomer, α,β-ethylenically unsaturated monocarboxylic acidmonomer, α,β-ethylenically unsaturated polyvalent carboxylic acidmonomer, α,βethylenically unsaturated polyvalent carboxylic acidanhydride monomer, aromatic vinyl monomer, fluorine-containing vinylmonomer, and copolymerizable anti-aging agent.

It is preferable that the nitrile group-containing copolymer rubber [A]further contain (meth)acrylic acid alkoxyalkyl ester monomer units (c),in addition to the α,β-ethylenically unsaturated nitrile monomer units(a1), the conjugated diene monomer units (b1), and the α,β-ethylenicallyunsaturated dicarboxylic acid monoester monomer units (d1). With thenitrile group-containing copolymer rubber [A] further including the(meth)acrylic acid alkoxyalkyl ester monomer units (c), it is possibleto obtain a cross-linked rubber that exhibits further improvements incold resistance and fuel oil resistance and has a more excellent balanceof fuel oil resistance and cold resistance.

The (meth)acrylic acid alkoxyalkyl ester monomer (mc) forming the(meth)acrylic acid alkoxyalkyl ester monomer units (c) is notparticularly limited as long as it is an ester compound composed of (i)an acrylic acid or a methacrylic acid and (ii) an alcohol having analkoxyalkyl group. Note that “(meth)acrylic acid” hereinafter means anacrylic acid and/or a methacrylic acid.

Examples of such a (meth)acrylic acid alkoxyalkyl ester monomer (mc)include methoxymethyl (meth)acrylate, methoxyethyl (meth)acrylate,ethoxymethyl (meth)acrylate, ethoxyethyl (meth)acrylate, n-propoxyethyl(meth)acrylate, i-propoxyethyl (meth)acrylate, n-butoxyethyl(meth)acrylate, i-butoxyethyl (meth)acrylate, t-butoxyethyl(meth)acrylate, methoxypropyl (meth)acrylate, and methoxybutyl(meth)acrylate. Among these compounds, the (meth)acrylic acidalkoxyalkyl ester monomer (mc) is preferably a compound containing analkoxyalkyl group having 2 to 8 carbon atoms, more preferably a compoundcontaining an alkoxyalkyl group having 2 to 6 carbon atoms, furtherpreferably methoxyethyl acrylate and ethoxyethyl acrylate, particularlypreferably methoxyethyl acrylate, and especially preferably2-methoxyethyl acrylate.

The percentage of the content of the (meth)acrylic acid alkoxyalkylester monomer units (c) in the nitrile group-containing copolymer rubber[A] is preferably 11 wt % to 30 wt %, more preferably 15 wt % to 25 wt%, and further preferably 20 wt % to 25 wt % of the total monomer unitsin the nitrile group-containing copolymer rubber [A]. In a case wherethe content of the (meth)acrylic acid alkoxyalkyl ester monomer units(c) is not less than 11 wt %, an obtained cross-linked rubber willexhibit further improvements in oil resistance and cold resistance. In acase where the content of the (meth)acrylic acid alkoxyalkyl estermonomer units (c) is not more than 30 wt %, an obtained cross-linkedrubber will exhibit improvements in fatigue resistance and sliding wearproperty.

Examples of the (meth)acrylic acid alkyl ester monomer include methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl(meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, andt-butyl (meth)acrylate.

Examples of the α,β-ethylenically unsaturated monocarboxylic acidmonomer include an acryl acid, a methacrylic acid, and a crotonic acid.

Examples of the α,β-ethylenically unsaturated polyvalent carboxylic acidmonomer include an itaconic acid, a fumaric acid, and a maleic acid.

Examples of the α,β-ethylenically unsaturated polyvalent carboxylic acidanhydride monomer include a maleic anhydride.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene,and vinylpyridine.

Examples of the fluorine-containing vinyl monomer includefluoroethylvinyl ether, fluoropropylvinyl ether, o-trifluoroethylene,vinyl pentafluorobenzoate, difluoroethylene, and tetrafluoroethylene.

The copolymerizable anti-aging agent is exemplified by, for example,N-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide,N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide,N-phenyl-4-(3-vinylbenzyloxy)aniline, andN-phenyl-4-(4-vinylbenzyloxy)aniline.

Two or more of these copolymerizable other monomers may be used incombination. The content of the other monomer units, which are not theα,β-ethylenically unsaturated nitrile monomer units (a1), the conjugateddiene monomer units (b1), the (meth)acrylic acid alkoxyalkyl estermonomer units (c), or the α,β-ethylenically unsaturated dicarboxylicacid monoester monomer units (d1), is preferably not more than 50 wt %,more preferably not more than 30 wt %, and further preferably not morethan 10 wt % of the total monomer units that constitute the nitrilegroup-containing copolymer rubber [A].

[Nitrile Group-Containing Copolymer Rubber [B]]

The nitrile group-containing copolymer rubber [B] contained in thecross-linkable rubber composition in accordance with the presentinvention is a nitrile group-containing copolymer rubber including notless than 30 wt % α,β-ethylenically unsaturated nitrile monomer units(a2) and the conjugated diene monomer units (b2), wherein at least partof the conjugated diene monomer units (b2) may be hydrogenated.Inclusion of the nitrile group-containing copolymer rubber [B] improvesfuel oil resistance.

(α,β-Ethylenically Unsaturated Nitrile Monomer Units (a2))

As an α,β-ethylenically unsaturated nitrile monomer (ma2) forming theα,β-ethylenically unsaturated nitrile monomer units (a2), any of thesame compounds as those listed above as the α,β-ethylenicallyunsaturated nitrile monomer (ma1) can be used. As the α,β-ethylenicallyunsaturated nitrile monomer (ma2), two or more of these compounds may beused in combination. Among these compounds, acrylonitrile andmethacrylonitrile are preferable, and acrylonitrile is particularlypreferable.

The percentage of the content of the α,β-ethylenically unsaturatednitrile monomer units (a2) in the nitrile group-containing copolymerrubber [B] is not less than 30 wt % and preferably not less than 35 wt %of the total monomer units in the nitrile group-containing copolymerrubber [B]. Further, the percentage of the content of theα,β-ethylenically unsaturated nitrile monomer units (a2) is preferablynot more than 90 wt %, more preferably not more than 70 wt %, andfurther preferably not more than 50 wt %. If the content of theα,β-ethylenically unsaturated nitrile monomer units (a2) is too low, anobtained cross-linked product tends to have decreased fuel oilresistance. If the content of the α,β-ethylenically unsaturated nitrilemonomer units (a2) is too high, an obtained cross-linked product tendsto have decreased cold resistance.

(Conjugated Diene Monomer Units (b2))

As a conjugated diene monomer (mb2) forming the conjugated diene monomerunits (b2), any of the same compounds as those listed above as theconjugated diene monomer (mb1) can be used. As the conjugated dienemonomer (mb2), two or more of these compounds may be used incombination. Among these compounds, 1,3-butadiene is preferable.

The percentage of the content of the conjugated diene monomer units (b2)is not more than 70 wt % and preferably not more than 65 wt % of thetotal monomer units in the nitrile group-containing copolymer rubber[B]. Further, the percentage of the content of the conjugated dienemonomer units (b2) is preferably not less than 10 wt %, more preferablynot less than 20 wt %, and further preferably not less than 30 wt %. Ifthe content of the conjugated diene monomer units (b2) is too low, aresulting cross-linked rubber tends to have poor rubber elasticity anddecreased cold resistance. In a case where the percentage of the contentof the conjugated diene monomer units (b2) is not more than 70 wt %,properties of an obtained cross-linked rubber, such as oil resistance,heat aging resistance, and chemical resistant stability, become good.

The nitrile group-containing copolymer rubber [B] is preferably suchthat at least part of the conjugated diene monomer units (b2) in thenitrile group-containing copolymer rubber [B] is hydrogenated. In thatcase, the nitrile group-containing copolymer rubber [B] has an iodinevalue of preferably not more than 120, more preferably not more than 80,further preferably not more than 25, and particularly preferably notmore than 10. In a case where the iodine value is not more than 120, anobtained cross-linked rubber will exhibit improvements in heatresistance and ozone resistance.

(Other Monomer Units)

The nitrile group-containing copolymer rubber [B] may contain othermonomer units copolymerizable with the α,β-ethylenically unsaturatednitrile monomer (ma2) and the conjugated diene monomer (mb2).

Examples of a monomer forming the other monomer units includeα,β-ethylenically unsaturated dicarboxylic acid monoester monomer (md2),(meth)acrylic acid alkoxyalkyl ester monomer (mc), (meth)acrylic acidalkyl ester monomer, α,β-ethylenically unsaturated monocarboxylic acidmonomer, α,β-ethylenically unsaturated polyvalent carboxylic acidmonomer, α,β-ethylenically unsaturated polyvalent carboxylic acidanhydride monomer, aromatic vinyl monomer, fluorine-containing vinylmonomer, and copolymerizable anti-aging agent.

The nitrile group-containing copolymer rubber [B] preferably containsα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d2), from the viewpoint that the nitrile group-containing copolymerrubber [B] is co-cross-linked by the nitrile group-containing copolymerrubber [A] so that a cross-linked rubber exhibits further improvementsin mechanical property and compression set resistance.

The percentage of the content of the α,β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d2) is preferably 4.5 to 10wt %, more preferably 6 to 9.5 wt %, and further preferably 6.5 to 8.5wt % of the total monomer units in the nitrile group-containingcopolymer rubber [B]. In a case where the content of theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d2) is not less than 4.5 wt %, an obtained cross-linked rubber willexhibit improvements in mechanical property and compression setresistance. Meanwhile, in a case where the content of theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d2) is not more than 10 wt %, a cross-linkable rubber compositionobtained by addition of a cross-linking agent will have good scorchstability, and an obtained cross-linked rubber will exhibit improvementin fatigue resistance.

As an α,β-ethylenically unsaturated dicarboxylic acid monoester monomer(md2) forming the α,β-ethylenically unsaturated dicarboxylic acidmonoester monomer units (d2), any of the same compounds as those listedabove as the α,β-ethylenically unsaturated dicarboxylic acid monoestermonomer (md1) can be used. As the α,β-ethylenically unsaturateddicarboxylic acid monoester monomer (md2), two or more of thesecompounds may be used in combination. Among these compounds, theα,β-ethylenically unsaturated dicarboxylic acid monoester monomer (md2)is preferably monopropyl maleate, mono n-butyl maleate, monopropylfumarate, mono n-butyl fumarate, monopropyl citraconate, mono n-butylcitraconate, or other monoester of a dicarboxylic acid in which twocarbon atoms forming an α,β-ethylenically unsaturated bond have eachcarboxyl groups, more preferably mono n-butyl maleate, monopropylcitraconate, or other monoester of a dicarboxylic acid having the twocarboxyl groups at the cis positions (cis arrangement), and particularlypreferably mono n-butyl maleate, from the viewpoint of allowing theadvantageous effect of the present invention to become more pronounced.

The content of the other monomer units, which are not theα,β-ethylenically unsaturated nitrile monomer units (a2), the conjugateddiene monomer units (b2), or the α,β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d2), is preferably not morethan 50 wt %, more preferably not more than 30 wt %, and furtherpreferably not more than 10 wt % of the total monomer units thatconstitute the nitrile group-containing copolymer rubber [B].

Note that the (meth)acrylic acid alkoxyalkyl ester monomer (mc), the(meth)acrylic acid alkyl ester monomer, the α,β-ethylenicallyunsaturated monocarboxylic acid monomer, the α,β-ethylenicallyunsaturated polyvalent carboxylic acid monomer, the α,β-ethylenicallyunsaturated polyvalent carboxylic acid anhydride monomer, the aromaticvinyl monomer, the fluorine-containing vinyl monomer, thecopolymerizable anti-aging agent, and others are in conformity with thedescriptions in the above section [Nitrile Group-Containing CopolymerRubber [A]].

[Polymer Mooney Viscosity]

A combination of the nitrile group-containing copolymer rubber [A] andthe nitrile group-containing copolymer rubber [B] (which can behereinafter referred collectively to as “nitrile group-containingcopolymer rubber”), which are contained in the cross-linkable rubbercomposition in accordance with the present invention, has a polymerMooney viscosity (ML_(1+4,) 100° C.) of preferably 15 to 200, morepreferably 15 to 150, and further preferably 15 to 100. With use of anitrile group-containing copolymer rubber having a polymer Mooneyviscosity of not less than 15, an obtained rubber cross-linked productwill have an excellent mechanical property. Meanwhile, with use of anitrile group-containing copolymer rubber having a polymer Mooneyviscosity of not more than 200, a cross-linkable rubber compositionobtained by adding a cross-linking agent to the nitrile group-containingcopolymer rubber has good processability.

[Method of Producing Nitrile Group-Containing Copolymer Rubbers]

A method of producing the nitrile group-containing copolymer rubbers [A]and [B] contained in the cross-linkable rubber composition in accordancewith the present invention is not particularly limited. Each of therubbers can be produced in the following manner. After copolymerizationof the above-described monomers is performed by emulsion polymerizationusing an emulsifier, unreacted monomers are removed if desired toprepare a latex of a nitrile group-containing copolymer rubber. Thelatex is hydrogenated as necessary, and then solidified and dried toobtain an intended solid rubber. At the time of emulsion polymerization,an emulsifier, a polymerization initiator, a molecular weight adjuster,or other usually used secondary polymerization material can be used.

Non-limiting examples of the emulsifier encompass nonionic emulsifiersincluding polyoxyethylene alkyl ether, polyoxyethylene alkyl phenolether, polyoxyethylene alkyl ester, and polyoxyethylene sorbitan alkylester; anionic emulsifiers including (i) salts of fatty acids such as amyristic acid, a palmitic acid, an oleic acid, and a linolenic acid,(ii) alkylbenzene sulfonate such as sodium dodecylbenzene sulfonate,(iii) higher alcohol sulfates, and (iv) alkyl sulfosuccinates; andcopolymerizable emulsifiers including sulfoester of α,β-unsaturatedcarboxylic acid, sulfate ester of α,β-unsaturated carboxylic acid, andsulfoalkyl arylether. The amount of the emulsifier used is preferably0.1 to 10 parts by weight, with respect to 100 parts by weight of thetotal monomers to be subjected to polymerization.

The polymerization initiator is not particularly limited as long as itis a radical initiator. Examples of the polymerization initiator includeinorganic peroxides such as potassium persulfate, sodium persulfate,ammonium persulfate, potassium perphosphate, and hydrogen peroxide;organic peroxides such as t-butyl peroxide, cumen hydroperoxide,p-mentane hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide,acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, dibenzoylperoxide, 3,5,5-trimethylhexanoyl peroxide, and t-butylperoxyisobutyrate; and azo compounds such as azobisisobutyronitrile,azobis-2,4-dimethyl valeronitrile, azobiscyclohexane carbonitrile, andmethyl azobisisobutyrate. These polymerization initiators can be usedalone or in combination of two or more thereof.

Among the exemplified polymerization initiators, an inorganic peroxideor an organic peroxide is preferable. The peroxide, when used as thepolymerization initiator, may be combined with a reducing agent such assodium bisulfite and ferrous sulfate, for use as a redox-basedpolymerization initiator. The amount of the polymerization initiatorused is preferably 0.01 to 2 parts by weight with respect to 100 partsby weight of the total monomers to be subjected to polymerization.

The molecular weight adjuster is not particularly limited. Examples ofthe molecular weight adjuster include mercaptans such as t-dodecylmercaptan, n-dodecyl mercaptan, and octyl mercaptan; halogenatedhydrocarbons such as carbon tetrachloride, methylene chloride, andmethylene bromide; a-methylstyrene dimer; and sulfur-containingcompounds such as tetraethylthiuram disulfide, dipentamethylene thiuramdisulfide, and diisopropyl xantogen disulfide. These compounds may beused alone or in combination of two or more thereof. Among thesecompounds, mercaptans are preferable, and t-dodecyl mercaptan is morepreferable. The amount of the molecular weight adjuster used ispreferably 0.1 to 0.8 part by weight with respect to 100 parts byweight' of the total monomers to be subjected to polymerization.

For example, water can be used as a medium of the emulsionpolymerization. The amount of water is preferably 80 to 500 parts byweight with respect to 100 parts by weight of the total monomers to besubjected to polymerization.

In performing the emulsion polymerization, a secondary polymerizationmaterial(s) such as a stabilizer, a dispersant, a pH adjuster, adeoxidizer, and a particle size adjuster can be further used ifnecessary. A type(s) and amount(s) of such a secondary polymerizationmaterial(s), if used, are not particularly limited.

Further, by selectively hydrogenating double bonds of the conjugateddiene monomer units (b1) of an obtained copolymer, it is possible toproduce the nitrile group-containing copolymer rubber [A]. In a casewhere the nitrile group-containing copolymer rubber [B] is to be used inwhich at least part of the conjugated diene monomer units (b2) ishydrogenated, it is possible to produce such a nitrile group-containingcopolymer rubber [B] by selectively hydrogenating double bonds of theconjugated diene monomer units (b2). Note that the type and amount of ahydrogenation catalyst used for the hydrogenation, a hydrogenationtemperature, and other conditions are determined in accordance with aknown method.

Methods of the hydrogenation include hydrogenation performed on a latexand hydrogenation performed on a solid rubber solution that is obtainedby dissolving, in an organic solvent, a solid rubber, which has beenobtained by solidifying and drying the latex. In the former case, it ispossible to obtain an intended solid rubber by subjecting the latex tothe hydrogenation reaction and then solidifying and drying the latexthus obtained. In the latter case, it is possible to obtain an intendedsolid rubber by removing the organic solvent, by steam stripping or thelike, from the solution having been subjected to hydrogenation reaction,and then drying a residue.

[Mixture Ratio of Nitrile Group-Containing Copolymer Rubbers [A] and[B]]

A mixture ratio (weight ratio) of the nitrile group-containing copolymerrubbers [A] and [B] ([A]/[B]) in the cross-linkable rubber compositionin accordance with the present invention is preferably 20/80 to 80/20,more preferably 25/75 to 75/25, and further preferably 40/60 to 60/40.In a case where the mixture ratio of the nitrile group-containingcopolymer rubber [A] is not less than 20 with respect to 100, which is asum of the amount of the nitrile group-containing copolymer rubber [A]and the amount of the nitrile group-containing copolymer rubber [B], asufficient degree of cold resistance will be exhibited. In a case wherethe mixture ratio of the nitrile group-containing copolymer rubber [B]is not less than 20 with respect to 100, which is a sum of the amount ofthe nitrile group-containing copolymer rubber [A] and the amount of thenitrile group-containing copolymer rubber [B], a sufficient degree ofoil resistance will be exhibited.

[Amine-based Cross-linking Agent]

The cross-linkable rubber composition in accordance with the presentinvention contains the amine-based cross-linking agent. This improvescompression set resistance and allows excellent cold resistance to beimparted to an obtained cross-linked rubber.

As specific examples of the amine-based cross-linking agent, theamine-based cross-linking agent is not particularly limited as long asit is a compound having two or more amino groups or is any material thatturns into the compound having two or more amino groups duringcross-linking. More specifically, the amine-based cross-linking agent ispreferably (i) a compound having an aliphatic hydrocarbon or aromatichydrocarbon with a plurality of hydrogen atoms substituted by aminogroups or by hydrazide structures (structures expressed by —CONHNH₂,where CO indicates a carbonyl group) and (ii) any material that turnsinto the (i) compound during cross-linking. Specific examples of theamine-based cross-linking agent include aliphatic polyvalent amines suchas hexamethylene diamine, hexamethylene diamine carbamate,tetramethylene pentamine, hexamethylene diamine cinnamaldehyde adduct,and hexamethylene diamine dibenzoate salt; aromatic polyvalent aminessuch as 2,2 -bis{4-(4-aminophenoxy)phenyl}propane, 4,4′-methylenedianiline, m-phenylene diamine, p-phenylene diamine, and 4,4′-methylenebis(o-chloroaniline); and compounds each having two or more hydrazidestructures, such as dihydrazide isophthalate, dihydrazide adipate, anddihydrazide sebacate. Among these compounds, aliphatic polyvalent aminesare preferable, and hexamethylene diamine carbamate is particularlypreferable.

The amount of the amine-based cross-linking agent to be mixed in thecross-linkable rubber composition in accordance with the presentinvention is preferably 0.1 part by weight to 20 parts by weight, morepreferably 0.2 part by weight to 15 parts by weight, further preferably0.5 part by weight to 10 parts by weight, particularly preferably 1 partby weight to 5 parts by weight, and most preferably 1.5 part by weightto 4 parts by weight, with respect to a total amount of 100 parts byweight of the combination of the nitrile group-containing copolymerrubbers [A] and [B]. In a case where the amount of the amine-basedcross-linking agent is not less than 0.1 part by weight, an obtainedcross-linked rubber will exhibit further improvements in mechanicalproperty and compression set resistance. Meanwhile, in a case where theamount of the amine-based cross-linking agent is not more than 20 partsby weight, an obtained cross-linked rubber will have good fatigueresistance.

[Basic Cross-linking Accelerator]

It is preferable that the cross-linkable rubber composition inaccordance with the present invention further contain a basiccross-linking accelerator, in addition to the nitrile group-containingcopolymer rubbers [A] and [B] and the amine-based cross-linking agent. Across-linkable rubber composition further containing the basiccross-linking accelerator will exhibit the advantageous effect of thepresent invention more pronouncedly.

Specific examples of the basic cross-linking accelerator include basiccross-linking accelerators having cyclic amidine structures such as1,8-diazabicyclo[5,4,0]undecene-7 (which can be hereinafter abbreviatedas “DBU”) and 1,5-diazabicyclo[4,3,0]nonene-5 (which can be hereinafterabbreviated as “DBN”), 1-methylimidazole, 1-ethylimidazole,1-phenylimidazole, 1-benzylimidazole, 1,2-dimethylimidazole,1-ethyl-2-methylimidazole, 1-methoxyethylimidazole,1-phenyl-2-methylimidazole, 1-benzyl-2-methylimidazole,1-methyl-2-phenylimidazole, 1-methyl-2-benzylimidazole,1,4-dimethylimidazole, 1,5-dimethylimidazole, 1,2,4-trimethylimidazole,1,4-dimethyl-2-ethylimidazole, 1-methyl-2-methoxyimidazole,1-methyl-2-ethoxyimidazole, 1-methyl-4-methoxyimidazole,1-methyl-2-methoxyimidazole, 1-ethoxymethyl-2-methyl imidazole,1-methyl-4-nitroimidazole, 1,2-dimethyl-5-nitroimidazole,1,2-dimethyl-5-aminoimidazole, 1-methyl-4-(2-aminoethyl)imidazole,1-methylbenzoimidazole, 1-methyl-2-benzylbenzoimidazole,1-methyl-5-nitrobenzoimidazole, 1-methylimidazoline,1,2-dimethylimidazoline, 1,2,4-trimethylimidazoline,1,4-dimethyl-2-ethylimidazoline, 1-methyl-phenylimidazoline,1-methyl-2-benzylimidazoline, 1-methyl-2-ethoxyimidazoline,1-methyl-2-heptylimidazoline, 1-methyl-2-undecylimidazoline,1-methyl-2-heptadecylimidazoline, 1-methyl-2-ethoxymethylimidazoline,and 1-ethoxymethyl-2-methylimidazoline; guanidine-based basiccross-linking accelerators such as tetramethylguanidine,tetraethylguanidine, diphenylguanidine, 1,3-di-o-tolylguanidine, ando-tolylbiguanide; and aldehydeamine-based basic cross-linkingaccelerators such as n-butylaldehyde aniline and acetoaldehyde ammonia.

Among these basic cross-linking accelerators, the guanidine-basedcross-linking accelerators and the basic cross-linking acceleratorshaving cyclic amidine structures are preferable,1,3-di-o-tolylguanidine, 1,8-diazabicyclo[5,4,0]undecene-7, and1,5-diazabicyclo[4,3,0]nonene-5 are further preferable, and1,8-diazabicyclo[5,4,0]undecene-7 is particularly preferable. Note thatthe basic cross-linking accelerators having cyclic amidine structuresmay form salts with, for example, organic carboxylic acids andalkylphosphoric acids.

The amount of the basic cross-linking accelerator to be mixed in thecross-linkable rubber composition in accordance with the presentinvention is 0.1 part by weight to 20 parts by weight, more preferably0.5 part by weight to 10 parts by weight, and further preferably 1 partby weight to 5 parts by weight, with respect to a total amount of 100parts by weight of the combination of the nitrile group-containingcopolymer rubbers [A] and [B]. In a case where the amount of the basiccross-linking accelerator is not less than 0.1 part by weight, across-linkable rubber composition will have an adequate cross-linkingrate and good cross-linking density. Meanwhile, in a case where theamount of the basic cross-linking accelerator is not more than 20 partsby weight, a cross-linking rate of a cross-linkable rubber compositionis prevented from getting too fast, resulting in preventing theoccurrence of scorching. Besides, good storage stability is achieved.

[Other Compounding Agents]

The cross-linkable rubber composition in accordance with the presentinvention may further contain a compounding agent that can be used inthe rubber field, in addition to the nitrile group-containing copolymerrubbers [A] and [B], the amine-based cross-linking agent, and theoptionally added basic cross-linking accelerator. Examples of such acompounding agent to be mixed include reinforcing fillers such as carbonblack and silica; nonreinforcing fillers calcium carbonate and clay;cross-linking accelerators other than the basic cross-linkingaccelerators; cross-linking aids; cross-linking retarders; anti-agingagents; antioxidants; photo-stabilizers; scorch preventers such asprimary amine; plasticizers; processing aids; slip agents; tackifiers;lubrication agents; flame retardants; antifungal agents; acid acceptors;antistatic agents; and pigments. The amounts of these compounding agentsare not particularly limited as long as they are mixed in a range thatwould not impair the object and advantageous effects of the presentinvention. The compounding agents can be mixed in any amounts accordingto purposes for mixing the compounding agents.

In the cross-linkable rubber composition in accordance with the presentinvention, another rubber that is not the nitrile group-containingcopolymer rubber [A] or [B] may be mixed in a range that would notimpair the advantageous effects of the present invention. The amount ofthe other rubber optionally mixed in the cross-linkable rubbercomposition is preferably not more than 30 parts by weight, morepreferably not more than 20 parts by weight, and further preferably notmore than 10 parts by weight, with respect to a total amount of 100parts by weight of the combination of the nitrile group-containingcopolymer rubbers [A] and [B].

[Preparation Method]

The cross-linkable rubber composition in accordance with the presentinvention is prepared by mixing the foregoing ingredients in anonaqueous system.

A method of preparing the cross-linkable rubber composition inaccordance with the present invention is not limited. For example, thecross-linkable rubber composition in accordance with the presentinvention can be prepared by the following method. The ingredients,except for the cross-linking agent and the thermally unstablecross-linking aid, etc., are primarily kneaded by a Bambury mixer, aninternal mixer, a kneader, or other mixer. Thereafter, the ingredientsare transferred to a roll kneading machine or other machine and thenmixed with the cross-linking agent and the thermally unstablecross-linking aid, etc. The mixture is secondarily kneaded to obtain thecross-linkable rubber composition in accordance with the presentinvention.

The thus obtained cross-linkable rubber composition in accordance withthe present invention has a compound Mooney viscosity [ML_(1+4,) 100°C.] that can be adjusted suitably to 15 to 200, more preferably 20 to175, and further preferably 25 to 150. The cross-linkable rubbercomposition in accordance with the present invention having such acompound Mooney viscosity is excellent in workability.

<Cross-linked Rubber>

A cross-linked rubber in accordance with the present invention contains:a nitrile group-containing copolymer rubber [A] including less than 30wt % α,β-ethylenically unsaturated nitrile monomer units (a1),conjugated diene monomer units (b1), and α,β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d1), wherein at least part ofthe conjugated diene monomer units (b1) is hydrogenated; and a nitrilegroup-containing copolymer rubber [B] including not less than 30 wt %α,β-ethylenically unsaturated nitrile monomer units (a2) and conjugateddiene monomer units (b2), wherein at least part of the conjugated dienemonomer units (b2) may be hydrogenated, the nitrile group-containingcopolymer rubber [A] and the nitrile group-containing copolymer rubber[B] being cross-linked with an amine-based cross-linking agent.

The cross-linked rubber in accordance with the present invention issuitably obtained from the cross-linkable rubber composition inaccordance with the present invention. The cross-linked rubber inaccordance with the present invention is therefore excellent in bothfuel oil resistance and cold resistance. Further, the present inventionencompasses a method for producing a cross-linked rubber, including:mixing a nitrile group-containing copolymer rubber [A], a nitrilegroup-containing copolymer rubber [B], and an amine-based cross-linkingagent, thereby cross-linking the nitrile group-containing copolymerrubber [A] and the nitrile group-containing copolymer rubber [B] withthe amine-based cross-linking agent, the nitrile group-containingcopolymer rubber [A] including less than 30 wt % α,β-ethylenicallyunsaturated nitrile monomer units (a1), conjugated diene monomer units(b1), and α,β-ethylenically unsaturated dicarboxylic acid monoestermonomer units (d1), wherein at least part of the conjugated dienemonomer units (b1) is hydrogenated, the nitrile group-containingcopolymer rubber [B] including not less than 30 wt % α,β-ethylenicallyunsaturated nitrile monomer units (a2) and conjugated diene monomerunits (b2), wherein at least part of the conjugated diene monomer units(b2) may be hydrogenated.

A more specific example method for producing the cross-linked rubber inaccordance with the present invention is as follows. That is, thecross-linked rubber in accordance with the present invention can beproduced, for example, by molding the cross-linkable rubber compositionin accordance with the present invention by a molding machine designedfor a desired shape, heating the cross-linkable rubber composition sothat a cross-linking reaction occurs, and then fixing the shape of thecross-linkable rubber composition to obtain a cross-linked product. Themolding machine is, for example, an extruder, an injection moldingmachine, a compressor, rolls, etc. In such a method, molding may beperformed in advance before cross-linking. Alternatively, molding andcross-linking may be performed simultaneously. A molding temperature ispreferably 10° C. to 200° C. and more preferably 25° C. to 120° C. Across-linking temperature is preferably 100° C. to 200° C. and morepreferably 130° C. to 190° C. A cross-linking time is preferably 1minute to 24 hours and more preferably 2 minutes to 6 hours.

There may be a case where, depending on the shape, size, etc. of anintended cross-linked rubber, an internal part of a cross-linkablerubber composition is insufficiently cross-linked even when a surface ofthe cross-linkable rubber composition is cross-linked. Just in case, thecross-linkable rubber composition may be further heated for itssecondary cross-linking.

As a heating method, a method usually used for cross-linking of rubber,such as press heating, steam heating, oven heating, or hot air heating,may be selected as appropriate.

The thus obtained cross-linked rubber in accordance with the presentinvention is excellent in both fuel oil resistance and cold resistanceand has an excellent balance of fuel oil resistance and cold resistance.Further, the cross-linked rubber in accordance with the presentinvention has good normal physical properties and is especiallyexcellent in compression set resistance.

As such, the cross-linked rubber in accordance with the presentinvention can be used for various types of seal materials such asO-rings, packings, diaphragms, oil seals, shaft seals, bearing seals,well head seals, air compressor seals, seals for sealingchlorofluorocabon or fluorohydrocarbons or carbon dioxide used forcooling apparatuses of air-conditioners or compressors for coolingmachines for air-conditioning systems, seals for sealing supercriticalcarbon dioxide or subcritical carbon dioxide used for washing media forprecision cleaning, seals for roller devices (e.g., a roller bearing, anautomotive hub unit, an automotive water pump, a linear guide device,and a ball screw), valves and valve seats, blowout preventers (BOP), andplatters; various types of gaskets such as an intake manifold gasketattached at a connecting part between an intake manifold and a cylinderhead, a cylinder head gasket attached at connecting part between acylinder block and a cylinder head, a rocker cover gasket attached atconnecting part between a rocker cover and a cylinder head, an oil pangasket attached at a connecting part between an oil pan and a cylinderblock or a transmission case, a fuel cell separator gasket attached inbetween a pair of housings sandwiching a unit cell provided with ananode, electrolyte plates, and cathode, and a top cover gasket of a harddisk drive; various types of rolls such as printing rolls, ironmakingrolls, papermaking rolls, industrial-use rolls, and office machineryrolls; various types of belts such as flat belts (e.g., a film core flatbelt, a cord flat belt, a laminated type flat belt, and a single unittype flat belt), V-belts (e.g., a lapped V-belt and a low edge

V-belt), V-ribbed belts (e.g., a single V-ribbed belt, a double V-ribbedbelt, a lapped V-ribbed belt, a back surface rubber V-ribbed belt, and atop cog V-ribbed belt), CVT belts, timing belts, toothed belts, andconveyor belts; various types of hoses such as fuel hoses, turbo airhoses, oil hoses, radiator hoses, heater hoses, water hoses, vacuumbrake hoses, control hoses, air-conditioner hoses, brake hoses, powersteering hoses, air hoses, marine hoses, risers, and flowlines; varioustypes of boots such as CVJ boots, propeller shaft boots, constantvelocity joint boots, and rack and pinion boots; damping material rubberparts such as cushion materials, dynamic dampers, rubber couplings, airsprings, and vibration isolators; dust covers; car interior members;tires; covered cables; shoe soles; electromagnetic wave shields;flexible printed circuit board binders and other binders; fuel cellseparators; and other wide range of applications in the fields ofcosmetics and pharmaceuticals, food-contacting fields, and electronicsfield, and other fields. Above all, the cross-linked rubber inaccordance with the present invention can be used suitably for belts,hoses, or seal materials and particularly suitably for seal materials.The cross-linked rubber in accordance with the present invention is usedsuitably for an O-ring among the seal materials.

The present invention is not limited to the embodiments described above,but can be altered in various ways within the scope of the appendedclaims. An embodiment derived from a proper combination of technicalmeans disclosed in different embodiments is also encompassed in thetechnical scope of the present invention.

EXAMPLES

The following description will discuss the embodiments of the presentinvention in more detail with reference to Examples. The presentinvention is, however, not limited to Examples described below. Notethat “part” mentioned in Examples below are based on weight unlessspecifically stated otherwise.

[Composition of Nitrile Group-containing Copolymer Rubber]

The percentages of contents of individual monomer units constituting thenitrile group-containing copolymer rubber were determined by ¹H-NMRanalysis and ¹³C-NMR analysis. Note that 1,3-butadiene units indicatedin Table 1 contain structural units obtained by hydrogenation ofcarbon-carbon double bonds in the 1,3-butadiene units.

[Iodine Value]

The iodine value of the nitrile group-containing copolymer rubber wasmeasured in accordance with JIS K 6235.

[Mooney Viscosity (Polymer Mooney)]

The Mooney viscosity of the nitrile group-containing copolymer rubberwas measured in accordance with JIS K 6300-1 (unit: [ML_(1+4,) 100°C.]).

[Fuel Oil Immersion Test]

Each cross-linked rubber was punched into a test piece having a size of30 mm×20 mm×2 mm. The test piece thus obtained was immersed at 40° C.for 168 hours in fuel oil (Fuel-C), which is a mixture of toluene andisooctane in a ratio of 50:50 by volume, to conduct a fuel oil immersiontest. Then, post-immersion volume swelling ΔV (unit: %) was calculatedin accordance with JIS K 6258. The smaller the post-immersion volumeswelling ΔV, the more excellent in fuel oil resistance.

[Cold Resistance Test]

The cold resistance of each cross-linked rubber was measured inaccordance with JIS K 6261 by a TR test (low temperature elasticityrecovery test). The lower a value of TR10(unit:° C.), the more excellentin cold resistance.

Plots of ΔV and TR10 obtained by the above two tests are placed on agraph (where an X-axis indicates ΔV, and an Y-axis indicates TR10). Inthe graph, a plot closer to the lower left part of the graph indicates amaterial having a good balance of fuel oil resistance and coldresistance.

Synthesis Example 1

One hundred and eighty parts of ion exchanged water, 25 parts of sodiumdodecylbenzene sulfonate aqueous solution with a 10 wt % concentration,23 parts of acrylonitrile (ma 1), 7 parts of mono n-butyl maleate (md1),30 parts of 2-methoxyethyl acrylate (mc), and 0.5 part oft-dodecylmercaptan (molecular weight adjuster) are placed in a metalbottle in this order. An internal gas in the metal bottle was thenreplaced with nitrogen three times. Subsequently, 40 parts of1,3-butadiene (mb1) was placed in the metal bottle. While the metalbottle was held at 5° C., 0.1 part of cumen hydroperoxide(polymerization initiator) was placed in the metal bottle. The mixturewas then allowed to make polymerization reaction for 16 hours while themetal bottle was kept being rotated.

Subsequently, 0.1 part of hydroquinone aqueous solution (polymerizationterminator) with a 10 wt % concentration was added to the mixture toterminate the polymerization reaction. Thereafter, a rotary evaporatorof 60° C. in water temperature was used to remove an unreacted monomer.In this way, a latex of a nitrile group-containing copolymer rubber(solid content concentration of approximately 30 wt %) was obtained.

Next, the latex thus obtained and a palladium catalyst (solution ofmixture of 1 wt % palladium acetate acetone solution and an equal weightof ion exchanged water) were added into an autoclave so that thepalladium content was 1,000 ppm with respect to the dry weight of therubber contained in the latex, and were subjected to a hydrogenationreaction at a hydrogen pressure of 3 MPa and a temperature of 50° C. for6 hours to obtain a latex of the nitrile group-containing copolymerrubber.

To the obtained latex of the nitrile group-containing copolymer rubber,methanol whose volume is twice higher than that of the latex was addedto solidify the latex. Thereafter, filtration was performed to extract asolid (crumb). The solid (crumb) was dried in vacuum at 60° C. for 12hours to obtain the nitrile group-containing copolymer rubber [A]. Thenitrile group-containing copolymer rubber [A] thus obtained had aniodine value of 7 and a polymer Mooney viscosity [ML_(1+4,) 100° C.] of50. The percentages of contents of monomer units constituting thenitrile group-containing copolymer rubber [A] thus obtained are shown inTable 1.

Synthesis Examples 2 and 3

Nitrile group-containing copolymer rubbers [B1] and [B2] were obtainedas in Synthesis Example 1 except that the amounts of acrylonitrile(ma2), mono n-butyl maleate (md2), 2-methoxyethyl acrylate (mc), and1,3-butadiene (mb2) to be used were changed to the amounts shown inTable 1.

The percentages of contents of monomer units constituting each of thenitrile group-containing copolymer rubbers thus obtained, iodine valuesthereof, and polymer Mooney viscosities thereof are shown in Table 1.

TABLE 1 Synthesis Examples 1 2 3 [A] [B1] [B2] Amounts of monomers usedAcrylonitrile (ma1) or (ma2) [part] 23 35 43 1,3-butadiene (mb1) or(mb2) [part] 40 59 57 Methoxyethyl acrylate (mc) [part] 30 0 0 Monon-butyl maleate (md1) or (md2) [part] 7 6 0 Nitrile group-containingcopolymer rubber Acrylonitrile units (a1) or (a2) [wt %] 24 36 441,3-butadiene units (b1) or (b2) [wt %] 46 58 56 2-methoxyethyl acrylateunits (c) [wt %] 23 0 0 Mono n-butyl maleate units (d) [wt %] 7 6 0Iodine value 7 10 7 Polymer Mooney viscosity (ML₁₊₄ 100° C.) 50 45 45

Example 1 Method of Preparation of Rubber Blend Sample Made of [A] and[B1] and Evaluation Results

A cross-linkable rubber composition was prepared, in accordance with theamounts of ingredients to be mixed shown in Table 2, by using a Bamburymixer and then using a roll kneading machine. Firstly, to the nitrilegroup-containing copolymer rubber [A] obtained in Synthesis Example 1and the nitrile group-containing copolymer rubber [B1] obtained inSynthesis Example 2, FEF carbon (product name “Seast SO”, manufacturedby Tokai Carbon, carbon black), tri-2-ethylhexyl trimellitate (productname “ADK Cizer C-8”, manufactured by ADEKA Corporation, plasticizer),4,4′-di-(α,α′-dimethylbenzyl)diphenylamine (product name “Noclac CD”,manufactured by Ouchi Shinko Chemical Industrial Co., Ltd., anti-agingagent), stearic acid, and polyoxyethylene stearylether phosphate(product name “Phosphanol RL210”, manufactured by Toho Chemical IndustryCo., Ltd., processing aid) were added and kneaded by using the Bamburymixer. Subsequently, the kneaded mixture was transferred to a rollkneading machine, and hexamethylene diamine carbamate (product name“Diak#1”, manufactured by DuPont Dow Elastomer LLC, polyamine-basedcross-linking agent) and 1,3-di-o-tolylguanidine (product name “NoccelarDT”, manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.,cross-linking accelerator) were further added thereto and kneaded toobtain a cross-linkable rubber composition.

Then, the cross-linkable rubber composition was placed in a mold of 15cm in length, 15 cm in width, and 0.2 cm in depth, and then subjected topress forming at 170° C. for 20 minutes by application of a presspressure of 10 MPa to obtain a cross-linked product in a sheet form.Subsequently, the cross-linked product thus obtained was transferred toa gear type oven and secondarily cross-linked at 170° C. for 4 hours.The cold resistance test and the fuel oil immersion test(toluene/isooctane of 50/50) were conducted on the cross-linked rubberthus obtained to perform evaluations on the cross-linked rubber. Theresults are shown in Table 2.

Example 2 Method of Preparation of Rubber Blend Sample Made of [A] and[B2] and evaluation results

A cross-linked rubber was obtained as in Example 1 except that thenitrile group-containing copolymer rubber [B1] was replaced by thenitrile group-containing copolymer rubber [B2]. Ingredients to be mixedand evaluation results are shown in Table 2.

Comparative Example 1 Method of Preparation of Rubber Blend Sample Madeof [B1] and [B2] and Evaluation Results

A cross-linked rubber was obtained as in Example 1 except that thenitrile group-containing copolymer rubber [A] was replaced by thenitrile group-containing copolymer rubber [B2]. Ingredients to be mixedand evaluation results are shown in Table 2.

Comparative Example 2 Method of Preparation of Rubber Sample Made Onlyof [A] and evaluation results

A cross-linked rubber was obtained as in Example 1 except that thecross-linked rubber was a rubber sample was made only of the nitrilegroup-containing copolymer rubber [A]. Ingredients to be mixed andevaluation results are shown in Table 2.

Comparative Example 3 Method of Preparation of Rubber Sample Made Onlyof [B1] and Evaluation Results

A cross-linked rubber was obtained as in Example 1 except that thecross-linked rubber was a rubber sample was made only of the nitrilegroup-containing copolymer rubber [B1]. Ingredients to be mixed andevaluation results are shown in Table 2.

Comparative Example 4 Method of Preparation of Rubber Sample Made Onlyof [B2] and Evaluation Results

A cross-linked rubber was obtained as in Example 1 except that thecross-linked rubber was a rubber sample was made only of the nitrilegroup-containing copolymer rubber [B2]. Ingredients to be mixed andevaluation results are shown in Table 2.

Based on numerical values in Table 2, plots of ΔV and TR10 are placed ongraphs (where X-axes indicate ΔV, and Y-axes indicate TR 10) shown inFIGS. 1 to 3. FIGS. 1 to 3 are graphs showing results of measurements ofcold resistance and fuel oil resistance in Examples and ComparativeExamples in accordance with the present invention.

From FIG. 1, it is apparent that plots for the rubber blends in Examples1-1 to 1-3 are closer to the lower left part of the graph than plots forthe single rubbers in Comparative Examples 2 and 3. This demonstratesthat blending rubbers leads to improvements in both fuel oil resistanceand cold resistance.

In FIG. 2, similarly to FIG. 1, the plots for the rubber blends inExamples 2-1 to 2-3 are closer to the lower left part of the graph thanplots for the single rubbers in Comparative Examples 2 and 4. Thisdemonstrates improvements in fuel oil resistance and cold resistance.

On the other hand, as shown in FIG. 3, plots for the rubber blends inComparative Example 1 are far from the lower left part of the graph.This demonstrates deteriorations in fuel oil resistance and coldresistance.

TABLE 2 Com. Com. Com. Com. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Example 1 Example 2[B1]/[B2] blend [A] [B1] [B2] [A]/[B1] blend [A]/[B2] blend Com. Com.Com. Com. Com. Com. Ex. 1-1 Ex. 1-2 Ex. 1-3 Ex. 2-1 Ex. 2-2 Ex. 2-3 Ex.1-1 Ex. 1-2 Ex. 1-3 Ex. 2 Ex. 3 Ex. 4 [A] 25 50 75 25 50 75 100 [B1] 7550 25 25 50 75 100 [B2] 75 50 25 75 50 25 100 Seast SO 45 45 45 45 45 4545 45 45 45 45 45 ADK Cizer C-8 5 5 5 5 5 5 5 5 5 5 5 5 Noclac CD 1.51.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Stearic acid 1 1 1 1 1 1 1 11 1 1 1 Phosphanol RL210 1 1 1 1 1 1 1 1 1 1 1 1 Diak#1 2.5 2.6 2.6 2.22.4 2.5 2.1 2.2 2.3 2.7 2.4 2 Noccelar DT 4 4 4 4 4 4 4 4 4 4 4 4Immersion test 55.3 55.8 56.5 43.7 48.7 53.9 42.4 47.0 50.9 58.0 54.639.6 (Fuel-C, 40° C. 168 hours) ΔV (%) TR test T10 (° C.) −25.3 −26.7−27.5 −18.0 −25.0 −27.0 −13.0 −14.0 −18.0 −28.0 −21.8 −12.3

INDUSTRIAL APPLICABILITY

The present invention is applicable to various rubber products.

The invention claimed is:
 1. A cross-linkable rubber compositioncomprising: a nitrile group-containing copolymer rubber [A] includingless than 30 wt % α, β-ethylenically unsaturated nitrile monomer units(a1), conjugated diene monomer units (b1), and α, β-ethylenicallyunsaturated dicarboxylic acid monoester monomer units (d1), wherein atleast part of the conjugated diene monomer units (b1) is hydrogenated; anitrile group-containing copolymer rubber [B] including not less than 30wt % α, β-ethylenically unsaturated nitrile monomer units (a2) andconjugated diene monomer units (b2), wherein at least part of theconjugated diene monomer units (b2) may be hydrogenated; and anamine-based cross-linking agent, the nitrile group-containing copolymerrubber [A] and the nitrile group-containing copolymer rubber [B] beingcontained in the cross-linked rubber at a weight ratio of 20/80 to80/20, and the content of the α, β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d1) in the nitrilegroup-containing copolymer rubber [A] is 6 wt % to 9.5 wt % of the totalmonomer units in the rubber.
 2. The cross-linkable rubber compositionaccording to claim 1, wherein the nitrile group-containing copolymerrubber [A] further includes (meth)acrylic acid alkoxyalkyl ester monomerunits (c).
 3. The cross-linkable rubber composition according to claim1, wherein the nitrile group-containing copolymer rubber [A] has aniodine value of not more than
 120. 4. The cross-linkable rubbercomposition according to claim 1, wherein the α, β-ethylenicallyunsaturated dicarboxylic acid monoester monomer units (d1) are units ofmono n-butyl maleate.
 5. A cross-linked rubber comprising: a nitrilegroup-containing copolymer rubber [A] including less than 30 wt % α,β-ethylenically unsaturated nitrile monomer units (a1), conjugated dienemonomer units (b1), and α, β-ethylenically unsaturated dicarboxylic acidmonoester monomer units (d1), wherein at least part of the conjugateddiene monomer units (b1) is hydrogenated; and a nitrile group-containingcopolymer rubber [B] including not less than 30 wt % α, β-ethylenicallyunsaturated nitrile monomer units (a2) and conjugated diene monomerunits (b2), wherein at least part of the conjugated diene monomer units(b2) may be hydrogenated, the nitrile group-containing copolymer rubber[A] and the nitrile group-containing copolymer rubber [B] beingcontained in the cross-linked rubber at a weight ratio of 20/80 to80/20, the nitrile group-containing copolymer rubber [A] and the nitrilegroup-containing copolymer rubber [B] being cross-linked with anamine-based cross-linking agent, and the content of the α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1) in the nitrile group-containing copolymer rubber [A] is 6 wt % to9.5 wt % of the total monomer units in the rubber.
 6. A method forproducing a cross-linked rubber, comprising: mixing a nitrilegroup-containing copolymer rubber [A], a nitrile group-containingcopolymer rubber [B], and an amine-based cross-linking agent at a weightratio of the nitrile group-containing copolymer rubber [A]/the nitrilegroup-containing copolymer rubber [B] in a range of 20/80 to 80/20,thereby cross-linking the nitrile group-containing copolymer rubber [A]and the nitrile group-containing copolymer rubber [B] with theamine-based cross-linking agent, the nitrile group-containing copolymerrubber [A] including less than 30 wt % α, β-ethylenically unsaturatednitrile monomer units (a1), conjugated diene monomer units (b1), and α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units(d1), wherein at least part of the conjugated diene monomer units (b1)is hydrogenated, the nitrile group-containing copolymer rubber [B]including not less than 30 wt % α, β-ethylenically unsaturated nitrilemonomer units (a2) and conjugated diene monomer units (b2), wherein atleast part of the conjugated diene monomer units (b2) may behydrogenated, and the content of the α, β-ethylenically unsaturateddicarboxylic acid monoester monomer units (d1) in the nitrilegroup-containing copolymer rubber [A] is 6 wt % to 9.5 wt % of the totalmonomer units in the rubber.